32 



THE EPITHELIAL TISSUES 



It is also found covering the mesentery and peritoneum in many 

 lower vertebrates in place of the simple squamous commonly found 

 in mammals. ^Yhen arranged about small ducts the cells are usu- 

 ally somewhat truncated with the free end smaller than the base. 

 Cuboidal cells in some situations, as in parts of the kidney tubules, 

 are often ciliated. 



A cuboidal epithelium is also found covering the o^•ary of 

 mammals, where it is spoken of as a germinal epithelium, though it 

 normally takes no part in germinal activity of the mature ovary. 



Simple Columnar Epithelium.— The cells of this type show varia- 

 tions from those whose length is only slightly greater than their 

 other two dimensions to cells which are greatly elongated. (Fig. 12.) 



Fig. 12. — Photograph of simple columnar epithelial colls forming the lining of 

 the intestine of Necturus. Several goblet cells are shown with their apical portions 

 filled with mucous secretion. 



The nuclei may occur in the central region of such cells, but more 

 commonly are found in the lower half. The low coliunnar cells 

 closely resemble the cuboidal and it is often difficult to decide 

 whether to call a given type cuboidal or low cohnnnar unless a 

 number of cells can be observed. In sections where the cut has 

 been parallel to the long axis, columnar cells appear as rectangles, 

 but sections at right angles to this axis show irregularly polygonal 

 figures. When serving to form ducts these cells nu\y ta])er from 

 base to apex, forming an elongated prism, or roughly pyramidal 

 figure in section. Epithelimn of a columnar form is found 



